1. Field of the Invention
The invention relates to optical disk drives, and more particularly to spherical aberration correction of optical disk drives.
2. Description of the Related Art
A perfect lens focuses all incoming rays to a point on the optic axis. A real lens with spherical surfaces, however, focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the axis. The real lens with spherical surfaces therefore does not produce a perfect focal point and suffers from spherical aberration.
An optical disk drive has a pickup head comprising a lens. The pickup head emits a laser beam, focuses the laser beam on a surface of an optical disk with the lens, and detects a reflection of the laser beam to read data stored on the surface of the optical disk. Because the lens of the pickup head is a real lens, the laser beam is not perfectly focused on the surface of the optical disk due to spherical aberration, inducing difficulties in data determination according to the reflection beam.
When an optical disk drive accesses conventional optical disks with a low data density such as DVDs, spherical aberration is ignored because the data density on the DVD disk and the required precision for data reading/writing are not so high, therefore the optical disk drive can correctly determines data according to refection beams with spherical aberration. When an optical disk drive accesses optical disks with a high data density such as HD-DVDs or Blu-ray Discs (BD), spherical aberration must be corrected. Otherwise, an optical disk drive cannot correctly determine data according to refection beams with spherical aberration. Thus, spherical aberration correction is required for optical disk drives accessing disks with high data density.
A conventional method for correcting spherical aberration is to move a collimator lens of a pickup head with a stepping motor. When the pickup head operates, heat is generated and the temperature is increased, causing damage to the mechanism of the stepping motor. If the mechanism of the stepping motor is severely damaged, the stepping motor cannot normally operate to correct spherical aberration, and the pickup head cannot correctly access data. Thus, a method for controlling spherical aberration correction with overheat protection is required.